1. Field of the Invention
This invention relates generally to the preparation of pseudo-binary alloys and relates more particularly to the preparation of homogenous, low-defect, single-crystal III-V alloys useful in high efficiency, multi junction solar cells.
2. Description of the Prior Art
The quality and efficiency of a solar cell in converting solar energy into electrical energy are largely determined by the cell materials and the cell configuration. The theoretical efficiency of a single-junction-configured cell, such as a single junction gallium arsenide (GaAs) cell, is only about 27.5%. To overcome this limitation the multi junction cascade or tandem solar cell has been proposed to obtain higher efficiencies by virtue of converting a greater portion of the solar spectrum than conventional cells. One approach which seem to be optimal, comprises a tandem cell of Group III-V alloys, which is in effect two solar cells in one. Each of the cells have having different band gaps arranged in such a way that one part of the solar spectrum is absorbed in a wide-band-gap material while the remaining part is absorbed in a narrow-band-gap material. The theoretical efficiency of such an arrangement is 36%. Discussion of such cells is included in a paper by J. C. Fan, B. Y. Tsaur, and B. J. Palm, entitled, "Optimal Design of High-Efficiency Tadem Cells," pp. Seventeenth IEEE Photovoltaic Specialists Conference, 692-701.
Materials that may prove suitable for such high-efficiency cells because from their outstanding properties, include semiconducting alloys formed from the compounds comprised of an element of Group III and an element of Group V of the Periodic Table particular interest are gallium aresenide phosphide "GaAs.sub.y P.sub.1-y " and gallium indium arsenide "Ga.sub.xIn.sub.1-x As". A great hindrance to developing this promising concept is the unavailability of suitable substrates upon which these materials can be grown. The principal requirement of the desired substrate is that it be highly matched in terms of lattice parameters with the cell layer. This is of great importance since mismatch is recognized as a major factor in limiting efficiency in a multijunction solar cell. Graded-layer epitaxy has been tried for growing the desired base cell on an available substrate, however this causes problems of interfacial dislocations. In addition, superlattice transitions and other approaches have been used, but mismatch problems persist.
In the III-V system, several ternary compositions including the above-mentioned ones are of great interest because they can give bottom cells with a 1.15 eV band gap and lattice-matched top cells with a 1.75 eV band gap. These gaps are near optimum for a two-cell PV converter and can combine to use enough of the solar spectrum to give a theoretical efficiency of about 36%.
No general process has heretofore been devised for satisfactorily producing bulk, single-crystal Group III-V pseudo-binary alloys of uniform composition.
A particular problem to be overcome in preparing single homogeneous crystals of pseudo-binary alloys stems from the inherent tendency of such alloys to form incongruently solidifying melts. For example, as a liquid mixture of a pseudo-binary alloy cools there will be a first-to-freeze part of the crystal that is substantially higher in one binary component than the last-to-freeze part of the crystal, giving a non-homogeneous crystal varying in composition over a wide range. This problem is more highly pronounced in materials of ternary III-V systems, which materials show solidus-liquidus curves with relatively large gaps between solidus and liquidus lines. Articles discussing such phenomena include J. W. Wagner 1970, "Preparation and Properties of Bulk In.sub.1-y Ga.sub.y As Alloys", J. Electrochem. Soc. Vol. 117, pp. 1193-1196; and F. Corrina, D. Margadonna, and P. Perfetti, 1973 "Growth of GaAs.sub.1-x P.sub.x Crystals by Pulling From Gallium-Rich Solutions", J. Crys. Growth 18, pp. 202-204. Such crystal alloys can be made more homogeneous by very rapid quenching of the melt, but this results in a highly polycrystalline structure made up of very small crystals. Solid state recrystallization processes have been proposed to convert such homogeneous polycrystals into a single crystal. One such process is described in U.S. Pat. No. 3,622,399 wherein a ternary alloy, in homogeneous polycrystalline form is held under a thermal gradient in order to recrystallize it into a single homogeneous crystal. This is ordinarily followed by an annealing of the bulk crystal. In practice, when such techniques are used with the aforementioned candidate alloys of GaInAs and GaAsP, they are more likely to produce not a monocrystal, but rather at best a consolidation of the polycrystals into a fewer number of larger homogeneous crystals. It is also noted that solid state recrystallization processes do not appear to lend themselves to economic production required in industry because of lengthy time periods ordinarily required for annealing and cooling.
One technique that has been used to produce homogeneous crystals is the so-called floating crucible system. The article by W. F. Leverton, 1958 "Floating Crucible Technique for Growing Uniformly Doped Crystals", J. Appl. Phys. Vol. 29, pp. 1241-1244, describes producing uniformly dopes germanium crystals by drawing them up from a germanium melt in a crucible that floats on a germanium melt in an outer, larger crucible. The first mentioned crucible has a narrow duct in the bottom. During the drawing-up process melt from the larger outer crucible can flow through the narrow duct. into the inner crucible. Such floating crucible systems have not been used to produce crystals of ternary III-V alloys.
There is another approach, shown in U.S. Pat. No. 3,305,485, that is specifically designed to produce single crystals from incongruently solidifying melts. A large supply crucible holds melt of the composition desired in the crystal. Spaced separately from the supply crucible is a small draw crucible, and a narrow tube extends from the supply vessel to the bottom of the draw crucible for conducting melt thereto. This approach is related to the floating crucible system to the extent that dual communicating crucibles are used; however it represents a deliberate departure from that system because of the concern that is stated in that disclosure regarding the adequacy of the floating crucible system in handling incongruently solidifying melts, particularly compounds with significant gaps between their solidus and liquidus lines.
In this regard it is noted that the floating crucible technique has been used for uniform growth and doping of (Bi.sub.0.05 Sb.sub.1.5)Te and Bi.sub.2 Te.sub.2.88 Se.sub.0.12. It has yet to be used with ternary III-V alloys such as those of the pseudo-binary systems GaP.sub.y As.sub.1-y and Ga.sub.x In.sub.1-x As which exhibit significantly larger gaps between their solidus and liquidus lines.